Following BREXIT from 1st January 2021 free movement no longer applies, so an EU citizen cannot travel to or enter the UK without applying for or being granted a UK immigration status. Depending on the length and purpose of entry to the UK, EU citizens may or may not be required to apply for a status in advance of travel.
Comprehensive information on all aspects of European Nationals visiting and working in the UK is available at The UK Visas and Immigration website